docs: def add_model(self, model):
          """
          Add a `RegressionModel` instance.
  
          Parameters
          ----------
          model : `RegressionModel`
              Should have a ``.name`` attribute matching one of
              the groupby segments.
  
          """
          logger.debug(
              'adding model {} to group {}'.format(model.name, self.name))
          self.models[model.name] = model